Surface acoustic wave sensors are often used to detect the presence of substances, such as chemicals. A surface acoustic wave or SAW device acting as a sensor provides a highly sensitive detection mechanism due to the high sensitivity to surface loading and the low noise, which results from their intrinsic high Q factor. Surface acoustic wave devices are fabricated using photolithographic techniques with comb-like interdigital transducers placed on a piezoelectric material. Surface acoustic wave devices may have either a delay line or a resonator configuration. The selectivity of a surface acoustic wave device sensor is generally determined by a selective coating placed on the piezoelectric material. The absorption and/or adsorption of the species to be measured into the selective coating causes mass loading, elastic, and viscoelastic effects on the device. The change of the acoustic property due to the absorption and/or adsorption of the species can be interpreted as a delay time shift for the delay line surface acoustic wave device or a frequency shift for the resonator surface acoustic wave device. However, the response of the surface acoustic wave sensor is also effected by environmental changes, such as temperature, pressure, stress, among others. These environmental changes degrade the response of the surface acoustic wave sensor. Temperature generally has the severest effect on the response, which may cause a misinterpretation. In the past, low temperature coefficient material has been selected to reduce the temperature effect. However, this has not always been successful because the selective coating used on the piezoelectric material may change the temperature characteristics of the material. Precision control of the temperature of the sensor has also been utilized, with temperature being controlled in the range of milidegrees. However, such precise temperature control is difficult, and temperature gradients between the sensor and a temperature sensor for the temperature control generally cannot be avoided. To separate the temperature effect from the measurand effect in surface acoustic wave sensors, the upper harmonic mode operation utilizing dispersion in the layered structures and the two device configuration with perpendicular direction as a convolver has been suggested. In bulk acoustic wave resonator sensors, the dual mode operation of a SC-cut quartz resonator was suggested for a temperature compensation. Temperature compensation in other devices is known. For example, in U.S. Pat. No. 4,535,638 entitled "Resonator Transducer System With Temperature Compensation" issuing to EerNisse et al on Aug. 20, 1985. Therein disclosed is an apparatus including an oscillator such as a quartz crystal, which is caused to resonate by the oscillator at two frequencies. The vibratory element is selected so that the two frequencies both vary with variations in force applied to the element and with variations in temperature of the element. Another device is disclosed in U.S. Pat. No. 5,869,763 entitled "Method For Measuring Mass Change Using A Quartz Crystal Microbalance" issuing to Vig et al on Feb. 9, 1999, which is herein incorporated by reference. Therein disclosed is a quartz crystal resonator excited in two different modes at the same time such that the mass change and the temperature change can be measured independently. The change in mass can be calculated accurately, independent of temperature effects.
Accordingly, there is a need to provide for temperature compensation in a surface acoustic wave sensor for detecting the presence of a substance or chemical.